PHOTOCHEMISTRY OF ORGANIC-MOLECULES ENTWINED IN SPIDERWEBS - THE USE OF POLY(METHYL METHACRYLATE) GLASS FOR RESTRICTING EXCITED-STATE MOTION

A study of several unimolecular photochemical rearrangements in poly(m ethyl methacrylate) glass was carried out with the intent of comparing the behavior of excited-state species trapped in a polymer matrix wit h both the corresponding solution photochemistry and that in crystal l attices. Three reactants were studied: (a) 4,5,5-triphenylcyclohex-2-e n-1-one, (b) 1,1-dicyano-3,3,5,5-tetraphenyl-1,4-pentadiene, and (c) i methyl-1,1-diphenyl-3-(2,2-diphenyl)cyclopropane. The solution photoch emistry of the three compounds had previously been studied in our labo ratory, and the photochemistry in the crystal lattice had been similar ly investigated for the first two. The crystal lattice photochemistry of the last of the three was investigated in the present study. For ea ch of the three reactants; different photochemical behavior was observ ed depending on the environment-polymer glass, solution, or crystal la ttice. The experimental behavior correlated nicely with theoretical as sessment of molecular motion of the component atoms in proceeding onwa rd from that point along the reaction coordinate where multiple reacti on pathways are available. The preferred reaction pathway proved consi stently to require the least motion and the minimum molecular volume d isplacement. A concept of least motion dependent on its application to the ''branch point'' rather than the entire conversion of reactant to product was established.